The present invention relates to aluminum alloy parts formed by forging aluminum alloy powder, such as orbiting and fixed scroll plates having involute wraps and the like, for use in a scroll-type compressor, and to a method of fabrication thereof.
Heretofore, when manufacturing spiral parts, such as orbiting and fixed scroll plates having involute wraps and the like, for use in a scroll-type compressor, the final finishing process has been performed by machining. The following methods have been used for preparing a shaped blank prior to final finishing: a casting method using cast-iron or a cast-aluminum alloy; an aluminum alloy die-casting method; a powder metallurgy method using iron sintered parts; a cold forging method using steel; and the like.
On the other hand, when manufacturing parts, such as connecting rods or the like, for use in a car, a powder forging technique has been used. The powder forging technique has not been practically use for aluminum parts, though it has been used for iron parts.
The aforementioned conventional manufacturing methods have the following disadvantages. The casting method using cast iron is disadvantageous in that the material used is heavy. Further, the accuracy of casting itself is so poor that machining is expensive. The machining time required is so long that cost cannot be reduced. Further, when thin parts are cast, defects such as blow holes and the like often arise.
The powder metallurgy method on iron sintered parts is disadvantageous in that the material used is heavy and inferior in airtightness because of the porosity (of the order of about ten per cent where the method is used for producing iron sintered parts). Further, the parts are so thin and spiral that high dimensional accuracy cannot be expected. Accordingly, it is difficult to reduce machining amount. Further, the machining of the parts must be carried out intermittently because of the presence of pores. Accordingly, machining speed cannot be increased.
The steel cold forging method is disadvantageous in that forging must be repeated to produce forged parts excellent in dimensional accuracy, so that cost cannot be reduced.
The aluminum alloy casting method and the die-casting method are disadvantageous in that an aluminum alloy to be used is limited to alloy compositions having good fluidity for thinning cast parts. Consequently, the thermal expansion coefficient of the cast aluminum alloy becomes relatively high and the Young's modulus thereof becomes relatively low, compared with an iron alloy. Further, it is difficult to maintain the strength and wear resistance at a predetermined level. Further, in the case where the Si content of the aluminum alloy to be used is high, the alloy cannot be machined at a high speed because of the coarse Si crystal grains, even though it may be possible to cast the alloy.